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The AAPG/Datapages Combined Publications Database
Houston Geological Society Bulletin
Abstract
Abstract: Geology and Hydrocarbons in Cook Inlet Basin, Alaska *
By
Thomas
E. Kelly
Executive Vice-President, Halbouty Alaska Oil Company
The Cook Inlet basin of south- central Alaska is an intermontane E structural basin approximately 14.000 square miles in area,
encompassing almost 80, 000 cubic miles of sedimentary rocks ranging in age
from Upper Triassic to Recent. The basin coincides with most of the northern part of the Matanuska geosyncline - an arcuate Mesozoic trough
located at the northwestern end of the Pacific Cordilleran mobile belt.
The Cook Inlet sedimentary trough, as contrasted with the structural
basin, is defined as a Paleozoic - early Mesozoic eugeosyncline that received
sediments from volcanic islands which were part of the volcanic archipelago
bordering the Pacific Coast of North America. Middle Jurassic epeirogeny
transformed southern Alaska into arcuate geanticlinal and geosynclinal belts
with the Cook Inlet basin beginning as a half graben created by complex faulting
on the east flank of the Talkeetna geanticline.
The Mesozoic ernbayment that collected marine sediments and occasional
nonmarine wedges abutting cratonic source areas was semienclosed or silled as
the Kenai and Chugach Ranges began to emerge following the early stages of the
Laramide orogeny. During the early Tertiary, an abundant supply of nonrnarine
clastic sediments and carbonaceous material was widely distributed in the subsiding intermontane basin.
The structural grain of the major tectonic elements describing the basin
architecture is preserved in trends of local structure. Intense folding and faulting
are exhibited on the north, east, and west flanks. Several major buried
anticlinal trends extend in a northeasterly direction through the interior.
End_Page 15------------------------- Figure 6. East-West Diagrammatic Structural Cross Section of Cook Inlet Basin, Alaska. Figure 7. Northeast-Southwest Diagrammatic Structural Cross Section of Cook Inlet
Basin, Alaska. End_Page 16------------------------- Mesozoic hydrocarbon accumulations associated with anticlinal traps are found on the western side of the basin. Minor quantities of oil, gas, and condensate
have been produced from sandstones of the Middle Jurassic Tuxedni formation. The oil is believed to be indigenous to Jurassic beds. Oil and gas accumulations in Tertiary beds will determine the significance of the Cook Inlet basin as an oil and gas province. Present oil production comes
from the Hemlock zone, a sandstone and conglomerate unit near the base of the Tertiary Kenai Formation. Entrapment has been influenced by folding and faulting
along trend of an interior basin high which lies adjacent and parallel to an Unnumbered Figure. Tectonic Map of Cook Inlet Basin, Alaska. End_Page 17------------------------- early Tertiary hinge belt. The Tertiary crude oils were probably derived from
Eocene marginal marine strata or from upper Cretaceous marine shales which
are unconformably overlain by the Tertiary sediments. Significant quantities of gas - predominantly methane - are present in the
loosely consolidated sands of the upper Kenai Formation. The two conditions
necessary for gas accumulation anywhere in the basin are (1) abundance of lignite
or coal beds in the section to serve as source rocks, and (2) a suitable trap. The Cook Inlet basin is in its earliest stage of exploration and development.
It is anticipated that many new fields will be discovered. Regional isopach maps
of the interval between the Mesozoic beds and the base of the Hemlock zone are
suggested as a basic approach to delineating old basin highs that may be sound
Hemlock prospects. Cook Inlet should become a major gas basin regardless of
its future as an oil province. End_of_Record - Last_Page 18-------- *Editor's Note: Mr. Kelly's talk before the Houston Geological Society on May
13 was based primarily on the above titled paper, first presented before the
AAPG at Denver, Colorado on April 27, 1961 and subsequently published in
Symposium Memoir #2, "The Backbone of the Americas - Tectonic History from
Pole to Pole. " However, the Tectonic Map (Fig. 1) published herewith has been
revised to show the results of developments subsequent to publication of the
original paper. As a matter of convenience to our readers the diagrammatic
cross sections of the Cook Inlet Basin (Figs. 6 & 7 of the original paper) are
also reproduced here.